1. Field of the Invention
The present invention is directed to a method for magnetic resonance imaging with adherence to SAR limits, wherein a patient is exposed to a radio-frequency pulse sequence via at least one transmission antenna for the implementation of a measurement in a magnetic resonance tomography apparatus and the magnetic resonance signals that are generated are acquired in a spatially resolved manner via at least one reception antenna and are further-processed for generating magnetic resonance images or spectra, and wherein current SAR values for planned parameters of the measurement are identified before the implementation of the measurement and the parameters are modified as needed until the current SAR values lie within the SAR limits. The invention also is directed to a magnetic resonance installation for the implementation of such a method.
2. Description of the Prior Art
Magnetic resonance tomography is a known technique for acquiring images of the inside of the body of an examination subject. For implementation of magnetic resonance tomography, a basic field magnet generates a static, relatively homogeneous basic magnetic field. Rapidly switched gradient fields for location coding that are generated by gradient coils, are superimposed on this basic magnetic field during the data acquisition for exposure of magnetic resonance images. Sequences of radio-frequency pulses for triggering magnetic resonance signals are emitted into the examination subject with one or more radio-frequency transmission antennas. The magnetic resonance signals occasioned by these radio-frequency pulses are produced by radio-frequency reception antennas. Tomograms of the inside of the body of the patient are calculated and displayed on the basis of the magnetic resonance signals received from the field of view (FoV) under observation, which may cover one or more body slices of the patient. All body regions from the head to the foot can be measured in this way by displacement of the patient bed within the magnetic resonance tomography apparatus.
A critical demand in modern magnetic resonance tomography is the capability for fast imaging. This demand results from economic considerations of being able to examine as many patients as possible per time interval and due to specific medical inquiries wherein a fast imaging is required for the examination result. One example of this is the reduction of motion artifacts due to movement of the patient during the measurement.
The high repetition rate of the radio-frequency transmission pulses or transmission pulse sequences required for a fast imaging, however, leads to a higher stress on the patient with electromagnetic radiation. Due to legal regulations, limit values are prescribed for this SAR (Specific Absorption Rate) stress that cannot be exceeded in magnetic resonance imaging. Since modern magnetic resonance tomography systems are technically capable of stressing patients with significantly higher SAR values than are legally permitted, SAR monitors are utilized in order to assure adherence to the limit values in the measurement. In addition to whole-body SAR values, specific limit values also must be adhered to for various body regions, for which a fundamental distinction must be made between whole body exposures, partial body exposures and local exposures.
German OS 40 42 212 discloses a method for magnetic resonance imaging wherein the primary magnetic field is cyclically switched between two field strengths in the implementation of a measurement in order adhere to the SAR limits and in order to utilize the latitude prescribed by the limit values as fully as possible. Detailed description relating to the determination of the SAR values are not provided in this publication.
The SAR stress is dependent on the individual patient data as well as on the position of the patient relative to the transmission antenna, on the type of transmission antenna, and the measurement parameters such as the transmission power, the repetition rate, the type of pulse sequence, the number of slices to be measures, and the position of the slices in the patient""s body. The parameters of the measurement usually are combined in a measurement protocol.
In order to prevent an upward transgression of the legally prescribed SAR limit values during the measurement, the SAR monitor measures the RF energy actually emitted by the system in order, if necessary, to shut off the RF system given an upward transgression of the legally allowed, accumulated RF energy within a predefined time interval. Additionally, a prediction (estimation) wherein the SAR values for the planned parameters of the measurement are determined, is implemented before every measurement. As a result of this prediction, the possibility of an upward transgression of the SAR limit values for the planned measurement is already recognized in advance, so that the measurement protocol can be varied as needed in order to adhere to the limit values. A prediction is based on determining that total energy at every point in time of the measurement time of a measurement protocol that respectively accumulates within the legally prescribed time interval for the respective SAR limit. This total energy in each time interval must not exceed the legally prescribed upper limit, the respective SAR limit value. Some of the legally prescribed limits are based on an averaging of the RF energy beamed in over a time span of a few minutes, for example 5 or 15 minutes.
In the conventional predictions that have hitherto been implemented, the parameters for the planned measurement, i.e. the measurement protocol, are employed in addition to the patient data for the determination of the current SAR values. Magnetic resonance measurements, however, often are shorter than the averaging times on which the SAR limits are based. Since a number of independent measurements are implemented on the patient in many instances, a maximum RF exposure conventionally has been assumed for safety reasons in the calculation of the current SAR values for the planned measurement, for a time interval before the beginning of the measurement that still lies within the averaging time for the particular SAR limit value. Even given, for example, very short, one-time measurements at the patient, however, this conservative prediction leads the actually permitted energy absorption being reduced by the quotient xe2x80x9crun time of the protocol/prescribed averaging time spanxe2x80x9d.
An object of the present invention is to provide a method as well as an apparatus for magnetic resonance imaging upon adherence to SAR limits that enable a better utilization of the performance of the magnetic resonance tomography apparatus in the imaging.
This object is achieved in a method and system of the type initially described wherein, according to the invention, the determination of the current SAR values for a planned measurement ensues using stored data that contain the time curve of the RF stressing of the patient during one or more preceding measurements as well as information from which the temporal spacing of the RF stress of the preceding measurements from the planned measurement can be determined. In the inventive method, thus, the actual RF stress of the patient in the past is taken into consideration for determining the averaging time required for the calculation of the SAR values insofar as preceding measurements still lie within this averaging time. A pre-condition for this is that suitable data are registered and stored in every measurement. Such data must contain the time curve of the RF stress on the patient as well as allowing a determination of the temporal spacing of the RF stress at an arbitrary point in time of the preceding measurements from an arbitrary point in time of the planned measurement. This can ensue by storing absolute time information with reference to the measured RF stresses or, for example, by the RF stress also being registered at defined, small time intervals in the pauses between individual, independent measurements, so that conclusions about the temporal spacing can be made from the measured data.
The appertaining magnetic resonance system must include a unit that records the RF power received in the patient during the measurement in terms of a time curve. Instead of the received power, the transmission power and the respective absorptionxe2x80x94the quotient of the power absorbed by the patient relative to the total transmission powerxe2x80x94can be logged separately from one another. This quotient varies with changes in the positioning of the patient relative to the transmission coil. Further, the unit for calculating the current SAR values must be configured such that it involves the stored data in the determination of these values.
In addition to containing the pure, planned measurement sequence, the prediction in the inventive method also contains the history immediately before the measurement. The previous performance history is taken into consideration in the prediction. The total averaging interval on which the corresponding SAR limit value is based is extended into the past for every arbitrary point in time of the planned measurement. This consideration is in turn implemented for every point in time of the measurement itself. Past and future therefore are no longer separated for a prediction but are united to form a continuous performance view.
An advantage of the inventive method is that the occurrence of a maximum exposure in the past no longer need be assumed for the determination of the current SAR values; rather, the actual RF stress that the patient was subjected to in the averaging time lying in the past is taken into consideration. In the case of a lower stress in the past, thus, a comparatively higher power can be emitted into the patient in the planned measurement without exceeding the allowed SAR limit values.